The present invention relates to an electronic part comprising a substrate and a conductive film formed inside the substrate.
Conventionally, it has been known that a chip type filter device incorporating capacitors and inductors serves as a preventive means against high frequency noises. Such a chip type filter device is manufactured by, for example, the following method.
Firstly, a dielectric sheet composed mainly of ceramic is prepared; a conductive film is formed on the dielectric sheet; by this, the dielectric sheet having the conductive film is manufactured. A number of such dielectric sheets carrying a conductive film are prepared; these dielectric sheets are placed one over another with a dielectric sheet carrying no conductive film inserted; and the assembly is pressed. Then, the dielectric sheets are bonded together by pressure, so that a lamination body having the stacked dielectric sheets is manufactured. The lamination body is sintered, and external electrodes are attached the sintered lamination body, so that a chip type filter device is manufactured.
FIG. 10 shows a schematic diagram of the cross-section of a part of the dielectric sheets of a filter device obtained by the above-described method. In the FIG. 10, two dielectric sheets 101 and 102 sandwiching a conductive film 100 (hatched part in the figure) are shown. As seen from the figure, the profile of conductive film 100 has a tapered shape at its both lateral ends 100a. Such a tapered shape is for this reason has a pressure applied onto the conductive film 100 when the stacked dielectric sheets 101 and 102 are pressed together. Therefore, the sectional area of the lateral end 100a of conductive film 100 is smaller than the sectional area of the center part 100b. 
When electric current flows through the conductive film 100, the current flowing through the internal part of the film 100 becomes comparatively small and current has a tendency to concentrate to the external surfaces of the film 100 due to the surface effect. This tendency is brought to the fore, as the frequency of current becomes high. Therefore, the higher the frequency of current becomes, the higher the current density at the lateral ends 100a becomes. In other words, a ratio of current flowing through the lateral ends 100a having a comparatively small cross-section is larger than ratio of current flowing through the center part 100b having a comparatively large cross-section. Because of this, the higher the frequency of current becomes, the higher the apparent resistance of the conductive film becomes, which results in a considerable ohmic loss of the high frequency components of current. Because of this property, the DC and low frequency components of current flow efficiently through the conductive film 100 while the high-frequency components of current are efficiently attenuated.
As seen from above, even if high frequency noises invade the conductive film 100, it is possible to efficiently attenuate the noises by preparing the film 100 having the small sectional area at its lateral ends 100a. Therefore, if the sectional area of the lateral ends 100a of conductive film 100 can be more reduced, it will be possible to more efficiently attenuate high-frequency noises. However, reducing the sectional area of the lateral ends 100a of conductive film 100 naturally has a limit, and, when the sectional area in question is close to the limit, it is difficult to further prevent the invasion of high-frequency noises.
With the above situation as a background, an object of the present invention is to provide an electronic part as described in the opening paragraph, which enables a further efficient attenuation of high-frequency noises.
The object is thereby achieved in that
the conductive film has lateral ends and an upper and a lower surface,
the substrate contains in peripheral parts at the lateral ends of the conductive film a first dielectric material,
the substrate contains in peripheral parts at the upper and lower surfaces of the conductive film a second dielectric material, and
the first dielectric material has a larger dielectric constant than the second dielectric material.
According to this invention, as described above, as to the dielectric materials constituting the substrate, first dielectric materials present in the peripheral parts of the lateral end of the conductive film have a higher dielectric constant than second dielectric materials present in the peripheral part of the upper and lower surfaces of the conductive film. Generally, if there is a dielectric material in contact with or close to the lateral end of a conductive film, the higher the dielectric constant of the dielectric material becomes, the more the tendency of concentrating electrical charge on the lateral end of conductive film accelerate. Therefore in the case of comparing an electronic part according to the present invention with an electronic part where the dielectric constant of the dielectric material is consistent across the entire part surrounding conductive film, the current density at the lateral end of the conductive film of the former electric part becomes higher than that of the latter electric part. Thus, in the case that the sectional area of the lateral end of conductive film is made smaller than the sectional area of the central part of the same film, when current flows through that conductive film, it is possible to efficiently flow the DC and low-frequency components of current through the film, and efficiently attenuate the high-frequencyxe2x80x94for example higher than 20 MHzxe2x80x94components of the same current.
In an advantageous embodiment, the substrate contains a first and a second electrode, which are insulated from the conductive film by the dielectric material present in the peripheral parts at the lower and the upper surfaces of the conductive film. Having two electrodes with dielectric material in between, the electronic part functionally contains a capacitor. The electronic part of this embodiment is for example an EMI-filter.
Preferably, the electrodes are cross-shaped. By further preference, the electrodes are stacked in between said dielectric material and another layer of dielectric material. This provides a good insulation. It is also advantageous, that the electronic part is surface mountable. This can be achieved with u-shaped terminations on at least two sides of the electronic part. If the electronic part is an EMI-filter, the terminations are present at two pairs of opposite sides of the electronic part. In this case the conductive film extends from the one to the other side of one pair of opposite sides, and the electrodes extend from the one to the other side of the other pair of opposite sides.
The ratio of the dielectric constants of the first and the second dielectric material is larger than 1, for example 2 or 10 or 20. In a further embodiment of the electronic part of the invention, the dielectric constant of the first dielectric material is at least five times as high as the dielectric constant of the second dielectric material. By having a difference of five an electronic part with an excellent attenuation of high-frequency noises is obtained. Examples of dielectric materials include SiN3, MgOxe2x80x94SiO2, CaOxe2x80x94MgOxe2x80x94TiO2, BaOxe2x80x94TiOxe2x80x94SnO2, BaOxe2x80x94MgOxe2x80x94CoOxe2x80x94Nb2O5, BaOxe2x80x94Nd2O3xe2x80x94TiO2xe2x80x94Bi2O3xe2x80x94Al2O3xe2x80x94MnO, BaOxe2x80x94MgOxe2x80x94Ta2O5, ZrO2xe2x80x94SnO2xe2x80x94PbOxe2x80x94CaOxe2x80x94ZrO2, CaOxe2x80x94MgOxe2x80x94TiO2, SrOxe2x80x94ZrO2xe2x80x94TiO2, BaOxe2x80x94Sm2O5xe2x80x94TiO2, PbOxe2x80x94BaOxe2x80x94Nd2O3xe2x80x94TiO2 and Li2Oxe2x80x94Na2Oxe2x80x94Sm2O5xe2x80x94TiO2xe2x80x94systems. Such materials with dielectric constants from 4 to 120 are known in the art of manufacturing ceramics. It has also been known, how to adapt the dielectric constant to a suitable value.